The present invention relates to a belt pretensioner for a vehicle seat belt system and, in particular, to a pretensioner for tightening the belt by rotating the belt reel shaft of a seat belt retractor in the belt-winding direction.
Some vehicle seat belt systems have a pretensioner for tightening a belt worn by an occupant of the vehicle in order to tightly restrain the body of the occupant in an emergency and to prevent the occupant from undergoing a secondary collision within the vehicle. Such a pretensioner is disclosed in Japanese Provisional Patent Publication No. 1-168547.
In the pretensioner of Japanese Publication No. 1-168547, a plurality of segmental arcuate pieces arranged in a circle around a belt reel shaft of a retractor are normally held in clearance from the shaft so that the retractor can function normally. When a cable wound around the pieces is pulled by a power source, the pieces are forced into engagement with the reel shaft and transmit displacement of the cable to the reel shaft such as to wind a portion of the belt onto the belt reel of the retractor. Although the pretensioner of JP 1-168547 requires relatively few components, inasmuch as the arcuate pieces are held in the "set" condition by the cable, the proper positioning of the pieces requires great care when the cable is wound around the pieces at assembly, lest the pieces be located too far from or too close to the belt reel shaft. Inaccurate positioning of the pieces can result in difficulties in the normal operation of the retractor due to interference between the pieces and the reel shaft or incorrect operation of the pretensioner when it operates, such as instability in the proper engagement of the pieces with the reel shaft.